Confused on offset vs hellical...shouldnt they offer a degree of helical too? Not just right or left?
I agree that their terminology is confusing (but not unusual). Any vane fletched at an offset angle to the shaft will by definition follow a helical path along the arrow. To be fully specified, their "helical" offering should include the angle between the base of the vane and the center of the arrow shaft, but I don't think I've ever seen helical fletching offered at a specified angle. I would interpret "helical" as equivalent to 4-5° offset.
Straight and/or offset vanes are applied using a jig with a straight clamp rotated to the specified angle (0-3° in this case) relative to the center axis of the arrow. In my experience, a 3° offset angle is about the practical limit when using a straight clamp. Beyond 3° it becomes difficult to get the front and back ends of the vane to both stay in contact with the shaft when gluing the vane on. A shorter vane or larger diameter shaft allows for greater offset angle with a straight clamp, but there's always going to be a limit. A "helical" clamp (sometimes called "true helical") extends that limit due to the curved shape of the clamp, which helps follow the curvature of the shaft and keep the vane in full contact when gluing it on. The
instruction manual for a Bitzenburger jig has some good visuals of straight vs. helical clamps and fletching technique. I haven't seen an angle advertised with helical fletching jigs/clamps, but I'd guess they achieve something in the neighborhood of a 4-5° offset.
can offset spin broadhead just fine though?
Yes, offset fletching will induce rotation of the arrow. The greater the offset angle, the faster the spin rate.
I guess just pick something and go for it?
Pretty much. If you like to tinker, get a Bitzenburger jig, fletch up several combinations of vane quantity/height/length/offset angle and shoot groups to find the most accurate combo. If you want to just make one choice and stick with it, you'll have to take an educated guess informed by general principles and the type of broadhead you plan to shoot (more BH surface area needs more fletching).
Drag force acting on the fletching is what enables the fletching to steer the arrow and compensate for imperfect shot execution, an out-of-tune bow, and the planing tendencies of broadheads. Drag force is proportional to surface area, so more vanes and/or longer/taller vanes will generate more drag and have greater steering ability. A greater degree of offset (or "helical") will also increase drag by orienting more of the vane's surface area perpendicular to the flight path. The downsides of more fletching are that drag slows the arrow down and that more vanes, taller vanes, and greater offset angles can cause clearance issues. Any contact between the fletching and cable, rest or sight housing is unacceptable, but that likely won't be an issue unless you're using exceptionally tall vanes.
As a point of reference, I've been able to get good broadhead-to-field point grouping out to 75 yds shooting 150 gr
Cutthroat broadheads with three 2" Blazer vanes at 3° offset out of a well-tuned bow.